The present invention relates to corrosion-protected ductile cast iron pipes, a method for the production thereof and also the use of specific compositions for producing corrosions-protected ductile cast iron pipes.
Ductile sleeved cast iron pipes have been used for a long time, e.g. for mains water and wastewater conduits. Ductile sleeved cast iron pipes are tubes made of cast iron and have to be provided with corrosion protection in order to increase their life owing to the fact that the tubes are in most cases laid underground when they are used. Ductile pipes laid underground are constantly in contact with a corrosive environment, i.e. the moist earth. In addition, during their use they come into contact with potentially corrosive materials, in the case of mains water, oxygen-containing water, in the case of mineral water, possibly acidic water, and in the case of wastewater, water which may contain further corrosive substances, e.g. of biological origin.
At present, it is mainly two methods which are used in the technical field to provide corrosion protection for ductile pipes.
The first method is based on the pipes being coated in a first step with an electrochemically active layer, usually of zinc, and in a second step with a second protective layer, e.g. composed of bitumen or of synthetic resin, with the latter usually consisting of epoxy resins or else of polyurethane. A modification of this method of providing corrosion protection consists of coating the pipes without prior zinc coating with a bitumen layer or a polymer layer which can consist of the abovementioned materials or else of polyethylene or the like.
Although this method has the advantage that the corrosion protection can be achieved using very thin layers, it at the same time has the disadvantage that the layers achieved in this way have a relatively low resistance to mechanical stresses, e.g. impacts but also cut-like damage. The use of very thin layers is advantageous with a view to the weight increase of the pipes and also further processing, in particular in the case of use of longitudinal adhesive sealing and connecting systems. The disadvantage reside in that increased care has to be taken when processing such pipes, since even damage to the corrosion protection in a relatively small region can lead to the pipe corroding within a considerably shorter time than the life expected and thus leaking or bursting. Furthermore, it has the disadvantage that considerable solvent emissions can sometimes arise during production. Coating with a polymer without prior zinc coating also has the disadvantage that the pipes have to be sand-blasted or chemically pickled in an appropriate way or pretreated in another complicated way after they have been produced in order to apply a firmly adhering corrosion protection, which makes the method more complicated and thus also more expensive.
A second method of providing corrosion protection on ductile cast iron pipes is application of a layer of a cement mortar, with an electrochemically active layer once again optionally being able to be applied, in particular on the outside between the cement mortar and the cast iron pipe.
Even though this method leads to coatings which have a significantly higher mechanical strength than is the case for polymer coatings, this method has the disadvantage that the layers required are relatively thick due to the required mechanical stability. A further disadvantage is the porosity of the cement mortar and that, for example when using coupling sleeve connections, the coating has to be removed again at the ends of the pipes, which once again significantly increases the processing costs.